The present invention relates to an electrical controller. The controller is typically connected to AC power lines or mains. The controllable current properties provided by the controller may be used to overcome electrical distortion in the power mains produced by an electrical element, such as an electrical load, connected to the controller or to the mains. The quality of the power in the AC mains is thereby maintained and the electrical efficiency in operating the load maximized. The controller has bidirectional power flow properties that permit it to function when the electrical element connected to the controller is either a load or an electrical power source.
Power quality is best and electrical operating efficiency greatest when the alternating current in the AC power mains, termed herein the "AC line current," is sinusoidal in wave form and in phase with the AC voltage in the mains, termed herein "line voltage." However, as noted in detail below, certain types of electrical elements, such as alternating current induction motors, shift the AC line current in the power mains out of phase with the AC line voltage. Other types of elements, such as AC to DC rectifier power supplies with a smoothing capacitor alter the sinusoidal waveform of the AC line current. While as a result of such alterations in the AC current, AC power quality and electrical efficiency has heretofore suffered, the controller of the present invention can establish the AC line current at an in phase, sinusoidal state, thereby obviating the foregoing shortcomings.
The above noted phase shift between AC current and line voltage is commonly described as low power factor. The term "power factor" is defined as the ratio of real power (measured in watts) to the apparent power (measured in volt-amperes). One cause of low power factor is an AC line current that lags the AC line voltage. A load which typically results in low power factor is an induction motor. When an induction motor is connected to AC power mains, the line current drawn by the motor is sinusoidal as is the line voltage, but the line current lags the line voltage.
In the prior art, the low power factor that characterizes an induction motor load has been corrected by placing a capacitor in parallel with the motor winding. The capacitor adds leading current to the lagging line current created by the motor's inductance, resulting in a near-zero phase displacement between the AC line current and line voltage.
A problem with this prior art approach, however, is that the capacitance of the capacitor is usually fixed whereas the electrical properties of the load may vary during operation. Thus, this approach does not satisfactorily correct the low power factor problem under all conditions.
Another problem with other types of loads is that, unlike induction motors, they draw non-sinusoidal current from the AC power mains. Fourier analysis can be used to resolve the non-sinusoidal current into a fundamental sinusoidal wave at line frequency and a number of multiples of line frequency known as harmonics. The fundamental waveform produces power in the load. However, the harmonic waveforms produce no net power in the load, and in fact result in heating losses, low power factor, and an inefficient use of the power distribution system. The harmonics add to the current demand on the AC power mains and typically result in a need for higher capacity wiring.
One example of an application which results in high harmonic content in the AC line current is a motor control using one or more silicon-controlled rectifiers (SCRs). SCR motor controls produce high levels of harmonics because the SCR switching is abrupt and results in discontinuous waveforms.
One prior art way of overcoming the problems arising from the presence of harmonics is the use of filters called harmonic wave traps. Such a wave trap is connected across the power mains supplying an SCR motor control that generates significant harmonic content. The wave trap typically includes filters for eliminating the 5th, 7th, 9th and 11th harmonics. However, the use of wave traps adds to the cost and complexity of the SCR equipment and is not always completely successful in eliminating harmonics.
An AC to DC rectifier power supply which has a smoothing capacitor is an example of another device which generates a high harmonic content in the AC line current. The AC power mains are connected to one side of a full wave rectifier bridge and the smoothing capacitor is connected across the DC side of the bridge. The capacitor voltage is applied to one or more voltage regulators that alter the capacitor voltage to the DC operating voltages needed for the equipment to be powered by the power supply. The regulators are typically of the switching type.
In a power supply of this type, the rectifier bridge connected to the AC power mains charges the capacitor with pulses of current that occur only at the peak of each half cycle of the AC line voltage. The AC line current is thus discontinuous and non-sinusoidal, resulting in harmonics, particularly at the points of discontinuity.
Since the current pulse is relatively narrow and must provide electrical power for the entire half cycle, the root-mean-square (RMS) value of the current pulse is much higher than would be the RMS value of sinusoidally-shaped current producing an equivalent amount of electrical power but extending continuously over the entire half cycle. This results in a reduction in the current and real power that may be obtained from the power supply for AC power mains of a given rating. For example, for AC power mains rated at 15 amps RMS, only approximately 8.5 amps of real power may be obtained from this type of power supply without exceeding the current rating of the mains. The pulse-like nature of the current also results in the reduction in the power factor of the power supply over that which would be obtained with a sinusoidal-shaped, continuous current.
The sharp current pulses additionally alter the voltage waveform in the AC power mains by reducing the voltage peaks due to the voltage drop caused by line impedance. This distortion of the AC line voltage may adversely effect other equipment connected to the line.
A further problem arising from the use of such power supplies is that of excessive loading of the neutral conductor when the AC power mains for the power supply comprises a three phase service. Under normal three phase service conditions, and when the currents are generally sinusoidal and electrical loads are balanced, there is little or no current flowing in the neutral conductor. This is due to the fact that current flowing in one phase is matched by an oppositely flowing current in one or both of the other phases.
However, due to the pulse-like nature of the current generated in this type of power supply, there may be no oppositely flowing current pulses in either of the other phases when current is flowing in one phase of a three phase service. The current must then flow through the neutral conductor. This problem becomes acute because this situation exists with respect to each of the three phases. Thus, the neutral conductor may be subjected to three times the thermal loading of any of the individual three phase lines. The high thermal loading may cause the neutral conductor to burn out creating a potential for fire. This is a serious concern in a situation in which, for example, an office building has a large number of computers having such power supplies installed.
As a result of the above-described problems, agencies which set standards for electrical power quality are beginning to establish requirements with respect to the amount of harmonic currents, and voltage waveform distortion, that will be permitted to be generated by the power supplies of electrical equipment.
In an attempt to reduce the AC line harmonics and to increase power factor, several types of power factor correction circuits have been developed. These power factor correction circuits are coupled in the DC portion of the power supply between the rectifier bridge and the output capacitor. The purpose of these circuits is to force the output current of the rectifier bridge to have a wave form which is shaped like a fully rectified sinusoid. The current reflected from the DC side, through the rectifier, to the AC side becomes essentially a sinusoidal AC current, thereby to reduce harmonics and increase power factor.
These prior art power factor correction circuits are generally of three types: buck, boost, and flyback (buck-boost). Each of these types of power correction circuits includes a current control element such as a power transistor. The current control element is operated in a pulse width modulated manner. The incremental direct current of the control element is established at a level that obtains the desired current during each switching increment. Capacitive, inductive, and diode elements smooth the current obtained during pulse width modulation to obtain a continuous, sinusoidal current.
These prior art power factor correction circuits have a number of limitations. One of these limitations is the nature of the current. For example, in a buck-type circuit, the input current is discontinuous, although the output current is continuous. The flyback-type circuit also has the discontinuous current at its input. These current discontinuities in the buck and flyback-type circuits often require use of an input filter that adds to the cost of the power supply.
Another problem with these prior art power factor correction circuits lies with the inductor in the circuit. Since this inductor is located at the DC output of the rectifier bridge, it is subjected to only unidirectional current. The inductor must therefore be sized to insure that it does not saturate under the anticipated operating conditions of the power supply. The result is that the inductor must often comprise a large and expensive component of the power supply.
A further shortcoming of the foregoing type of prior art power supplies is that they do not permit the bidirectional transfer of power between the AC power mains and the electrical element connected to the power supply. They thus cannot be used to supply electrical elements, such as motor drives where the motor must regenerate, or supply power back to the AC power mains.